This invention relates generally to the field of immunotherapy and, more specifically, to methods for enhancing host immune responses.
The immune system of mammals has evolved to protect the host against the growth and proliferation of potentially deleterious agents. These agents include infectious microorganisms such as bacteria, viruses, fungi, and parasites which exist in the environment and which, upon introduction to the body of the host, can induce varied pathological conditions. Other pathological conditions may derive from agents not acquired from the environment, but rather which arise spontaneously within the body of the host. The best examples are the numerous malignancies known to occur in mammals. Ideally, the presence of these deleterious agents in a host triggers the mobilization of the immune system to effect the destruction of the agent and, thus, restore the sanctity of the host environment.
The destruction of pathogenic agents by the immune system involves a variety of effector mechanisms which can be grouped generally into two categories: innate and specific immunity. The first line of defense is mediated by the mechanisms of innate immunity. Innate immunity does not discriminate among the myriad agents that might gain entry into the host""s body. Rather, it responds in a generalized manner that employs the inflammatory response, phagocytes, and plasma-borne components such as complement and interferons. In contrast, specific immunity does discriminate among pathogenic agents. Specific immunity is mediated by B and T lymphocytes and it serves, in large part, to amplify and focus the effector mechanisms of innate immunity.
The elaboration of an effective immune response requires contributions from both innate and specific immune mechanisms. The function of each of these arms of the immune system individually, as well as their interaction with each other, is carefully coordinated, both in a temporal/spatial manner and in terms of the particular cell types that participate. This coordination results from the actions of a number of soluble immunostimulatory mediators or xe2x80x9cimmune system stimulatorsxe2x80x9d (Reviewed in, Trinchieri, et al., J. Cell. Biochem. 53:301-308 (1993)). Certain of these immune system stimulators initiate and perpetuate the inflammatory response and the attendant systemic sequelae. Examples of these include, but are not limited to, the proinflammatory mediators tumor necrosis factors xcex1 and xcex2, interleukin-1, interleukin-6, interleukin-8, interferon-xcex3, and the chemokines RANTES, macrophage inflammatory proteins 1-xcex1 and 1-xcex2, and macrophage chemotactic and activating factor. Other immune system stimulators facilitate interactions between B and T lymphocytes of specific immunity. Examples of these include, but are not limited to, interleukin-2, interleukin-4, interleukin-5, interleukin-6, and interferon-xcex3. Still other immune system stimulators mediate bidirectional communication between specific immunity and innate immunity. Examples of these include, but are not limited to, interferon-7, interleukin-1, tumor necrosis factors xcex1 and xcex2, and interleukin-12. All of these immune system stimulators exert their effects by binding to specific receptors on the surface of host cells, resulting in the delivery of intracellular signals that alter the function of the target cell. Cooperatively, these mediators stimulate the activation and proliferation of immune cells, recruit them to particular anatomical sites, and permit their collaboration in the elimination of the offending agent. The immune response induced in any individual is determined by the particular complement of immune system stimulators produced, and by the relative abundance of each.
In contrast to the immune system stimulators described above, the immune system has evolved other soluble mediators that serve to inhibit immune responses (Reviewed in, Arend, W. P., Adv. Int. Med. 40:365-394 (1995)). These xe2x80x9cimmune system inhibitorsxe2x80x9d provide the immune system with the ability to dampen responses in order to prevent the establishment of a chronic inflammatory state with the potential to damage the host""s tissues. Regulation of host immune function by immune system inhibitors is accomplished through a variety of mechanisms as described below.
First, certain immune system inhibitors bind directly to immune system stimulators and, thus, prevent them from binding to plasma membrane receptors on host cells. Examples of these types of immune system inhibitors include, but are not limited to, the soluble receptors for tumor necrosis factors xcex1 and xcex2, interferon-xcex3, interleukin-1, interleukin-2, interleukin-4, interleukin-6, and interleukin-7.
Second, certain immune system inhibitors antagonize the binding of immune system stimulators to their receptors. By way of example, interleukin-1 receptor antagonist is known to bind to the interleukin-1 membrane receptor. It does not deliver activation signals to the target cell but, by virtue of occupying the interleukin-1 membrane receptor, blocks the effects of interleukin-1.
Third, particular immune system inhibitors exert their effects by binding to receptors on host cells and signalling a decrease in their production of immune system stimulators. Examples include, but are not limited to, interferon-xcex2, which decreases the production of two key proinflammatory mediators, tumor necrosis factor-xcex1 and interleukin-1 (Coclet-Ninin et al., Eur. Cytokine Network 8:345-349 (1997)), and interleukin-10, which suppresses the development of cell-mediated immune responses by inhibiting the production of the immune system stimulator, interleukin-12 (D""Andrea, et al., J. Exp. Med. 178:1041-1048 (1993)). In addition to decreasing the production of immune system stimulators, certain immune system inhibitors also enhance the production of other immune system inhibitors. By way of example, interferon-xcex12b inhibits interleukin-1 and tumor necrosis factor-xcex1 production and increases the production of the corresponding immune system inhibitors, interleukin-1 receptor antagonist and soluble receptors for tumor necrosis factors xcex1 and xcex2 (Dinarello, C. A., Sem. in Oncol. 24(3 Suppl. 9):81-93 (1997).
Fourth, certain immune system inhibitors act directly on immune cells, inhibiting their proliferation and function, thereby, decreasing the vigor of the immune response. By way of example, transforming growth factor-xcex2 inhibits a variety of immune cells, and significantly limits inflammation and cell-mediated immune responses (Reviewed in, Letterio and Roberts, Ann. Rev. Immunol. 16:137-161 (1998)). Collectively, these various immunosuppressive mechanisms are intended to regulate the immune response, both quantitatively and qualitatively, to minimize the potential for collateral damage to the host""s own tissues.
In addition to the inhibitors produced by the host""s immune system for self-regulation, other immune system inhibitors are produced by infectious microorganisms. For example, many viruses produce molecules which are viral homologues of host immune system inhibitors (Reviewed in, Spriggs, M. K., Ann. Rev. Immunol. 14:101-130 (1996)). These include homologues of host complement inhibitors, interleukin-10, and soluble receptors for interleukin-1, tumor necrosis factors xcex1 and xcex2, and interferons xcex1, xcex2 and xcex3. Similarly, helminthic parasites produce homologues of host immune system inhibitors (Reviewed in, Riffkin, et al., Immunol. Cell Biol. 74:564-574 (1996)), and several bacterial genera are known to produce immunosuppressive products (Reviewed in, Reimann, et al., Scand. J. Immunol. 31:543-546 (1990)). All of these immune system inhibitors serve to suppress the immune response during the initial stages of infection, to provide advantage to the microbe, and to enhance the virulence and chronicity of the infection.
A role for host-derived immune system inhibitors in chronic disease also has been established. In the majority of cases, this reflects a polarized T cell response during the initial infection, wherein the production of immunosuppressive mediators (i.e., interleukin-4, interleukin-10, and/or transforming growth factor-xcex2 dominates over the production of immunostimulatory mediators (i.e., interleukin-2, interferon-xcex3, and/or tumor necrosis factor-xcex2) (Reviewed in, Lucey, et al., Clin. Micro. Rev. 9:532-562 (1996)). Over-production of immunosuppressive mediators of this type has been shown to produce chronic, non-healing pathologies in a number of medically important diseases. These include, but are not limited to, diseases resulting from infection with: 1) the parasites, Plasmodium falciparum (Sarthou, et al. Infect. Immun. 65:3271-3276 (1997)), Trypanosoma cruzi (Reviewed in, Laucella, et al. Revista Argentina de Microbiologia 28:99-109 (1996)), Leishmania major (Reviewed in, Etges and Muller, J. Mol. Med. 76:372-390 (1998)), and certain helminths (Riffkin, et al., supra); 2) the intracellular bacteria, Mycobacterium tuberculosis (Baliko, et al., FEMS Immunol. Med. Micro. 22:199-204 (1998)), Mycobacterium avium (Bermudez and Champsi, Infect. Immun. 61:3093-3097 (1993)), Mycobacterium leprae (Sieling, et al. J. Immunol. 150:5501-5510 (1993)), Mycobacterium bovis (Kaufmann, et al., Ciba Fdn. Symp. 195:123-132 (1995)), Brucella abortus (Fernandes and Baldwin, Infect. Immun. 63:1130-1133 (1995)), and Listeria monocytogenes (Blauer, et al., J. Interferon Cytokine Res. 15:105-114 (1995)); and, 3) the intracellular fungus, Candida albicans (Reviewed in, Romani, et al., Immunol. Res. 14:148-162 (1995)). The inability to spontaneously resolve infection is influenced by other host-derived immune system inhibitors as well. By way of example, interleukin-1 receptor antagonist and the soluble receptors for tumor necrosis factors xcex1 and xcex2 are produced in response to interleukin-1 and tumor necrosis factor xcex1 and/or xcex2 production driven by the presence of numerous infectious agents. Examples include, but are not limited to, infections by Plasmodium falciparum (Jakobsen, et al. Infect. Immun. 66:1654-1659 (1998), Sarthou, et al., supra), Mycobacterium tuberculosis (Balcewicz-Sablinska, et al., J. Immunol. 161:2636-2641 (1998)), and Mycobacterium avium (Eriks and Emerson, Infect. Immun. 65:2100-2106 (1997)). In cases where the production of any of the aforementioned immune system inhibitors, either individually or in combination, dampens or otherwise alters immune responsiveness before the elimination of the pathogenic agent, a chronic infection may result.
In addition this role in infectious disease, host-derived immune system inhibitors contribute also to chronic malignant disease. Compelling evidence is provided by studies of soluble tumor necrosis factor receptor type I (sTNFRI) in cancer patients. Nanomolar concentrations of sTNFRI are synthesized by a variety of activated immune cells in cancer patients and, in many cases, by the tumors themselves (Aderka et al., Cancer Res. 51: 5602-5607 (1991); Adolf and Apfler, J. Immunol. Meth. 143: 127-36 (1991)). In addition, circulating sTNFRI levels often are elevated significantly in cancer patients (Aderka, et al., supra; Kalmanti, et al., Int. J. Hematol. 57: 147-152 (1993); Elsasser-Beile, et al., Tumor Biol. 15: 17-24 (1994); Gadducci, et al., Anticancer Res. 16: 3125-3128 (1996); Digel, et al., J. Clin. Invest. 89: 1690-1693 (1992)), decline during remission and increase during advanced stages of tumor development (Aderka, et al., supra; Kalmanti, et al., supra; Elsasser-Beile, et al., supra; Gadducci, et al., supra) and, when present at high levels, correlate with poorer treatment outcomes (Aderka, et al., supra). These observations suggest that sTNFRI aids tumor survival by inhibiting anti-tumor immune mechanisms which employ tumor necrosis factors xcex1 and/or xcex2 (TNF), and they argue favorably for the clinical manipulation of sTNFRI levels as a therapeutic strategy for cancer.
Direct evidence that the removal of immune system inhibitors provides clinical benefit derives from the evaluation of Ultrapheresis, a promising experimental cancer therapy (Lentz, M. R., J. Biol. Response Modif. 8: 511-27 (1989); Lentz, M. R., Ther. Apheresis 3: 40-49 (1999); Lentz, M. R., Jpn. J. Apheresis 16: 107-14 (1997)). Ultrapheresis involves extracorporeal fractionation of plasma components by ultrafiltration. Ultrapheresis selectively removes plasma components within a defined molecular size range, and it has been shown to provide significant clinical advantage to patients presenting with a variety of tumor types. Ultrapheresis induces pronounced inflammation at tumor sites, often in less than one hour post-initiation. This rapidity suggests a role for preformed chemical and/or cellular mediators in the elaboration of this inflammatory response, and it reflects the removal of naturally occurring plasma inhibitors of that response. Indeed, immune system inhibitors of TNF xcex1 and xcex2, interleukin-1, and interleukin-6 are removed by Ultrapheresis (Lentz, M. R., Ther. Apheresis 3: 40-49 (1999)). Notably, the removal of sTNFRI has been correlated with the observed clinical responses (Lentz, M. R., Ther. Apheresis 3: 40-49 (1999); Lentz, M. R., Jpn. J. Apheresis 16: 107-14 (1997)).
Ultrapheresis is in direct contrast to more traditional approaches which have endeavored to boost immunity through the addition of immune system stimulators. Pre-eminent among these has been the infusion of supraphysiological levels of TNF (Sidhu and Bollon, Pharmacol, Ther. 57: 79-128 (1993)), and of interleukin-2 (Maas, et al., Cancer Immunol. Immunother. 36: 141-148 (1993)), which indirectly stimulates the production of TNF. These therapies have enjoyed limited success (Sidhu and Bollon, supra; Maas, et al., supra) due to the fact: 1) that at the levels employed they proved extremely toxic; and, 2) that each increases the plasma levels of the immune system inhibitor, sTNFRI (Lantz, et al., Cytokine 2: 402-406 (1990); Miles, et al., Brit. J. Cancer 66: 1195-1199 (1992)). Together, these observations support the utility of Ultrapheresis as a biotherapeutic approach to cancerxe2x80x94one which involves the removal of immune system inhibitors, rather than the addition of immune system stimulators.
Although Ultrapheresis provides advantages over traditional therapeutic approaches, there are certain drawbacks that limit its clinical usefulness. Not only are immune system inhibitors removed by Ultrapheresis, but other plasma components, including beneficial ones, are removed since the discrimination between removed and retained plasma components is based solely on molecular size. An additional drawback to Ultrapheresis is the significant loss of circulatory volume during treatment, which must be offset by the infusion of replacement fluid. The most effective replacement fluid is an ultrafiltrate produced, in an identical manner, from the plasma of non-tumor bearing donors. A typical treatment regimen (15 treatments, each with the removal of approximately 7 liters of ultrafiltrate) requires over 200 liters of donor plasma for the production of replacement fluid. The chronic shortage of donor plasma, combined with the risks of infection by human immunodeficiency virus, hepatitis A, B, and C or other etiologic agents, represents a severe impediment to the widespread implementation of Ultrapheresis.
Because of the beneficial effects associated with the removal of immune system inhibitors, there exists a need for methods which can be used to specifically deplete those inhibitors from circulation. Such methods ideally should be specific and not remove other circulatory components, and they should not result in any significant loss of circulatory volume. The present invention satisfies these needs and provides related advantages as well.
The present invention provides a method for stimulating immune responses in a mammal through the depletion of immune system inhibitors present in the circulation of said mammal. The depletion of immune system inhibitors can be effected by removing biological fluids from said mammal and contacting these biological fluids with a binding partner capable of selectively binding to the targeted immune system inhibitor.
Binding partners useful in these methods can be antibodies, both polyclonal or monoclonal antibodies, or fragments thereof, having specificity for a targeted immune system inhibitor. Additionally, binding partners to which the immune system inhibitor naturally binds may be used. Synthetic peptides created to attach specifically to targeted immune system inhibitors also are useful as binding partners in the present methods. Moreover, mixtures of binding partners having specificity for multiple immune system inhibitors may be used.
In a particularly useful embodiment, the binding partner is immobilized previously on a solid support to create an xe2x80x9cabsorbent matrixxe2x80x9d (FIG. 1). The exposure of biological fluids to such an absorbent matrix will permit binding by the immune system inhibitor, thus, effecting a decrease in its abundance in the biological fluids. The treated biological fluid can be returned to the patient. The total volume of biological fluid to be treated and the treatment rate are parameters individualized for each patient, guided by the induction of vigorous immune responses while minimizing toxicity. The solid support (i.e., inert medium) can be composed of any material useful for such purpose, including, for example, hollow fibers, cellulose-based fibers, synthetic fibers, A flat or pleated membranes, silica-based particles, or macroporous beads.
In another embodiment, the binding partner can be mixed with the biological fluid in a xe2x80x9cstirred reactorxe2x80x9d (FIG. 2). The binding partner-immune system inhibitor complex then can be removed by mechanical or by chemical or biological means, and the altered biological fluid can be returned to the patient.
The present invention also provides apparatus incorporating either the absorbent matrix or the stirred reactor.